A heat pipe has been known as a mechanism in which a working fluid is sealed within a closed space and highly efficient heat transfer is performed by using the vaporization and condensation thereof.
In the heat pipe, a working fluid, which has received heat, is vaporized at a vaporizing portion, moves within a tubular path, and is then cooled at a condensing portion to thereby return to a liquid state. The condensed working fluid is again transported to the vaporizing portion (reflux or reflow).
There are many cases where the heat transport efficiency of a heat pipe is determined by the rate of the reflux. Therefore, in order to accelerate the reflux, there have been performed an attempt in which the flow path is directed downward to utilize gravity and an attempt in which a wick is provided to utilize a capillary force.
Among such attempts, heat pipes of a type in which working fluid is circulated within a loop-shaped closed space are called loop heat pipes. These pipes have been studied mainly for cosmic purposes. These heat pipes are called CPL (Capillary Pumped Loop), LHP (Loop Heat pipe), and the like.
The loop heat pipe includes a vaporizer, a condenser, a reserver, a loop pipe connecting these elements, and a working fluid for circulating within the pipe.
In the loop heat pipe, since there is no necessity to allow a flow path to be directed downward for hastening the reflux, the limitation of orientation thereof is relaxed as compared to a linear heat pipe using a linear pipe. Moreover, the degree of freedom of the shape thereof can be improved.
In such a loop heat pipe, it is important to allow a working fluid to flow in a predetermined direction without a back flow in a loop flow path.
For this reason, in Japanese Patent Application Laid-Open No. H07-332881, there are provided a forced circulation flow generating means (electromagnetic pump or the like) for preventing the back flow and a flow rate adjusting means (flow rate adjusting valve) for adjusting the circulation flow rate of a working fluid.
Moreover, in Japanese Patent Application Laid-Open No. 2003-148882, there is adopted a configuration in which a fluid reservoir portion for preventing the back flow of vapor is provided so that a working fluid is caused to flow in a predetermined direction.
As a method of storing a fluid within the fluid reservoir portion, there are proposed a method in which there is provided, within a flow path, a valve of a shape memory alloy such that when vapor comes into contact with a valve, the valve is heated and closed to prevent the back flow, and a method in which there is provided a filter to absorb condensed fluid by a capillary force.
Moreover, when an attempt is made to reduce the size of a heat pipe, as the size becomes smaller, the influence of a frictional force or surface tension of a pipe path becomes larger than the influence of gravity.
Namely, it is more advantageous to utilize the capillary force for the reflux than gravity. For example, in D. Liepmann, Proceedings of 2001 ASME International Mechanical Engineering Congress and Exposition November 11-16, New York (2001), a micro-loop heat pipe is produced utilizing semiconductor processing technology. Further, gravity is not used for the reflux, and the acceleration of the movement of the condensed fluid is realized by means of a wick provided at a vaporizing portion.
Fuel cells of various types have been studied and developed. Among the fuel cells, a polymer electrolyte fuel cell (proton exchange membrane fuel cell) has been widely studied and developed as an automotive or residential power generator, because it has a relative low operating temperature and uses a polymer film electrolyte, which is easy to handle.
Moreover, for the purpose of carrying and using a small-sized electric device, there have been used various primary batteries and secondary batteries.
However, with the recent development of high-performance small-sized electric devices, power consumption has become large, so that sufficient energy cannot be supplied by means of a primary battery without increasing the size and weight of the devices.
Moreover, while the secondary battery can be advantageously used by being repeatedly charged, the amount of energy that it can generate from a single charge is less than that of the primary battery.
Further, another power source is required for the purpose of charging a secondary battery. In addition, it generally takes from several tens of minutes to several hours to charge a secondary battery, so that it is difficult to allow the secondary battery to be immediately used on demand.
In the future, with the increase in the tendency of carrying and using electric devices, a further reduction in their size and weight, and implementation of wireless network environments, it is difficult for conventional primary and secondary batteries to supply a sufficient amount of energy to drive such devices.
Small-sized fuel cell have attracted attention as a countermeasure for such problems. The reason why a fuel cell is useful as a drive source for a small-sized electric device is that the amount of energy that can be supplied per unit volume or per unit weight is about several times to ten times that of a conventional battery.
Further, by only supplying fuel, the fuel cell can be continuously used, which avoids the delay associated with the charging of conventional secondary batteries.
A polymer electrolyte fuel cell using hydrogen as fuel or a direct methanol fuel cell is mainly used as such a small-sized fuel cell.
In a fuel cell for obtaining a large output, it is effective to use hydrogen as fuel.
However, hydrogen is gaseous at ordinary temperature, and it has been difficult to store hydrogen at a high density in a small fuel tank.
In view of the above, when hydrogen is to be used as fuel, in order to efficiently and safely store hydrogen, there is employed a method of filling a hydrogen storage alloy in a fuel tank and allowing hydrogen to be adsorbed by the alloy. When a hydrogen storage alloy is used, the reaction of releasing hydrogen is generally an endothermic reaction. For example, LaNi5 known as a hydrogen storage alloy absorbs heat of about 30 kJ when releasing 1 mole of hydrogen.
Moreover, the relationship between the temperature T of a hydrogen storage alloy and the hydrogen dissociation pressure PH2 is expressed by the following formula called “van't Hoff's equation:
      ln    ⁢                  ⁢          P              H        ⁢                                  ⁢        2              =                              2          ⁢          Δ          ⁢                                          ⁢                      H            0                          nR            ⁢              1        T              -                  2        ⁢        Δ        ⁢                                  ⁢                  S          0                    nR      wherein n is number of moles, and R is gas constant, and in LaNis, ΔH0=−30.1 (kJ/mol H2) and ΔS0=−108.8 (kJ/mol H2). As is seen from the above equation, as the hydrogen is released, the temperature of the fuel tank is lowered, and the pressure inside the tank and the hydrogen release rate are reduced.
Particularly, during power generation of a fuel cell, as hydrogen release and power generation proceed, the tank temperature decreases and the hydrogen release rate is reduced.
To the contrary, when the fuel tank is heated, the pressure inside the tank and the hydrogen release rate increase. Accordingly, in order to obtain a sufficient hydrogen release rate and to prevent the tank pressure from excessively increasing, the temperature inside the tank needs to be kept constant.
Power generation of a polymer electrolyte fuel cell is performed in a manner as described below.
As a polymer electrolyte membrane, perfluorosulfonic acid cation-exchange resin is frequently used. For example, Nafion (trade name; manufactured by DuPont Company) is well known as such a material.
A polymer electrolyte membrane is interposed between a pair of porous electrodes each carrying a catalyst, such as platinum, i.e., a fuel electrode and an oxidizer electrode constitute a membrane electrode assembly as a power generation cell.
An oxidizer is supplied to the oxidizer electrode of the fuel cell, and a fuel is supplied to the fuel electrode, so that protons move within the polymer electrolyte membrane to perform power generation.
Such a power generation reaction is most effective when performed within the temperature range of about 60° C. to about 100° C.
However, the polymer electrolyte membrane has such a property that when the temperature exceeds 100° C., the power generation performance is remarkably decreased. Moreover, although the polymer electrolyte membrane is generally used in a wet state, water in the polymer electrolyte membrane will be vaporized at a temperature of 100° C. or more.
Accordingly, it is not preferable for the power generation cell temperature to reach 100° C. or more during power generation.
The power generation efficiency of a polymer electrolyte fuel cell is about 50%, and heat in an amount that is approximately identical to the power generation amount is produced. Accordingly, in power generation, it is necessary to maintain fuel cell units at a suitable temperature.
In view of the above, in Japanese Patent Application Laid-Open No. 2004-31096, there is proposed a fuel cell in which heat produced by the power generation cell is radiated by using a fuel tank casing.
In this fuel cell, in order to prevent the temperature of the fuel tank from being excessively raised by the heat from the power generation cell, the casing and inside of the tank are separated by a heat insulating member.
Moreover, Japanese Patent Application Laid-Open No. H06-260202 discloses a method in which a heat exchange between a fuel cell unit and a fuel tank is performed by using cooling water. Also, Japanese Patent Application Laid-Open No. H10-064567 discloses a method in which a heat exchange between a fuel cell unit and a fuel tank is performed by using exhaust gas.
Further, as a method of efficiently performing a heat exchange directly between a fuel cell unit and a fuel tank without using a medium such as cooling water or exhaust gas, U.S. Pat. No. 6,268,077 discloses a fuel cell system as described below.
In this patent, there is disclosed a fuel cell system, which includes a fuel cell having power generation cells disposed in a planar form and a fuel tank, wherein a principal surface of the fuel cell and a principal surface of the fuel tank are in contact with each other.
Moreover, Japanese Patent Application Laid-Open No. 2000-353536 proposes a fuel cell using a heat pipe for radiation. In this fuel cell, for attaining a reflux in which a condensed working fluid is again transported to a vaporizing portion, there is employed a system in which a flow path using a linear heat pipe is directed downward to utilize a gravity.
Moreover, the theoretical voltage of one membrane electrode assembly set is about 1.23V, and there are many cases where the assembly is used at about 0.7 V in an ordinary operating state.
For this reason, when a higher voltage is required, or when a high output density is required, there is used a stack structure in which a plurality of fuel cell units are stacked to electrically connect the respective fuel cell units in series.
Such a stack structure is called a fuel cell stack. Generally, an anode flow path and a cathode flow path in the stack are separated by a member called a separator.
Generally, in the fuel cell stack, there is a tendency in that as a cell is positioned closer to the central part, its radiation efficiency becomes poor and its temperature becomes high, and as a cell is positioned closer to the end part, its temperature becomes low.
When a temperature difference is generated between the cells in the stack, a variance in power generation performance is caused depending on the power generation cell units, which is not preferable.
In view of the above, in Japanese Patent Application Laid-Open No. 2004-31096 mentioned above, there is employed a method in which a member having a high thermal conductivity is used as a member constituting a fuel cell (stack) to thereby prevent heat from residing in the vicinity of the central part of the stack to thereby reduce a temperature difference between fuel cell units.
However, the above-mentioned conventional examples of the loop heat pipe have problems as described below.
For example, the loop heat pipe disclosed in Japanese Patent Application Laid-Open No. H07-332881 is provided with the pump for preventing back flow.
However, when such a fluid machine is provided, the system becomes complicated and large, and the power consumption is increased, which results in disadvantages in achieving a reduction of the size of the system.
Moreover, when the valve made of a shape memory alloy or the fluid reservoir using a filter for absorbing condensed fluid by a capillary force is used as disclosed in Japanese Patent Application Laid-Open No. 2003-148882, the back flow of vapor can be prevented without using electric power.
However, because gravity is utilized to reflux the condensed fluid, there is a possibility that if the heat pipe is reduced in size so that the influence of a frictional force becomes large, or if the heat pipe is incorporated into a device the vertical direction of which is not determined, the heat pipe would not effectively operate.
Further, the valve made of a shape memory alloy is provided in a flow path of the heat pipe. However, this valve is opened/closed by the temperature of working vapor and cannot be controlled according to the temperature conditions of the outside of the pipe.
Furthermore, in the micro-pipe of D. Liepmann, Proceedings of 2001 ASME International Mechanical Engineering Congress and Exposition November 11-16, New York (2001), no gravity is used for refluxing. The movement of the condensed fluid can be accelerated by using wicks provided at a vaporizing portion. However, a mechanism for preventing the back flow is not incorporated. In addition, the quantity of transported heat cannot be controlled.
Further, the above-mentioned examples of the conventional fuel cell system have problems as described below. For example, the fuel cell of Japanese Patent Application Laid-Open No. 2004-31096 can prevent the temperature elevation of the fuel cell, but does not effectively function with respect to lowering the temperature inside the fuel tank.
Furthermore, since a heat transfer is performed mainly by heat conduction in a solid, when a large amount of heat is generated, or when the heat transport distance is large, there is a possibility that the ability to transport heat may become insufficient.
Further, in the fuel cells of Japanese Patent Application Laid-Open Nos. H06-260202 and H10-064567, when heat exchange is performed between the fuel cell unit and the fuel tank, a circulating device for cooling water or exhaust gas is required, which may result in a size increase of the system and a decrease in the energy utilization efficiency of the entire system.
The fuel cell disclosed in U.S. Pat. No. 6,268,077 does not require a special system for heat exchange, but the heat exchange amount is determined by the area of the fuel cell principal surface so that it is difficult to optimally control the heat exchange quantity depending on the temperature.
In addition, in Japanese Patent Application Laid-Open No. 2000-353536, for attaining reflux in which a condensed working fluid is again transported to a vaporizing portion, there is employed a system in which a flow path using a linear heat pipe is directed downward to utilize gravity.
Accordingly, when such a linear heat pipe utilizing gravity while being directed downward is used for a fuel cell the vertical direction of which at the time of use is not determined, there is a possibility that the heat pipe may not effectively operate.